Colored polymer coatings are used to protect and enhance the appearance of many different types of surfaces. Due to their durability, polymer coatings are often employed where the surface will be subject to prolonged wear or exposure to the elements and sunlight. In such applications it is essential that the coating retain its color despite the conditions to which it is exposed.
Colored polymer coatings are widely used in the automotive industry. The majority of cars manufactured today are painted with a "top coat system" which employs one or more layers of polymer coating. In such a system the bare metal of the auto body is first treated with a primer, usually an epoxy primer. The primer is then covered with a "basecoat" polymer layer that confers color and may also contain metal flakes if a metallic finish is desired. The base coat is then covered with a transparent "topcoat" or "overcoat" polymer layer that protects the basecoat from the elements. The topcoat may also contain an ultraviolet radiation absorber which helps to protect the basecoat from ultraviolet light.
It is preferred that such coatings are "lightfast" (i.e., exhibit stable coloration after prolonged exposure to light). Unfortunately, many coloring materials, including dyes and pigments, are degraded to varying degrees by prolonged light exposure. With time, colored coatings containing such materials will lose their color as the coloring material deteriorates, thus diminishing the aesthetic appeal of the coating. Other colored coating preparations grow darker or even change color with light exposure producing unwanted changes in appearance of the finish. Some protection is provided by incorporating an ultraviolet absorber in the topcoat layer. Although an absorber stops a significant portion of ultraviolet light from reaching the colored base coat, enough ultraviolet light makes it past the absorber that degradation of the coloring agent in the base coat is still a problem.
Traditional topcoat systems have for the most part relied on particulate pigments in the basecoat layer for coloring. The pigment is supplied in the form of opaque particles of associated pigment molecules which are suspended in the basecoat polymer. The pigment molecules on the outside of each particle contribute to coloring of the coating, while other molecules buried inside the particles are hidden and do not contribute to coloration of the coating. As one pigment molecule on the outside of a given particle is degraded by light and becomes dissociated from the particle, another molecule in the next layer of the particle is then exposed and can contribute to coloring the coating. As a result, although individual molecules of pigment are not lightfast, the total number of pigment molecules which contribute to coloration of the coating is maintained relatively constant.
Despite this advantage, however, pigments have demonstrated a legion of disadvantages for use in making polymer coatings. Since the majority of pigment molecules are buried within the interior of particles, pigments require use of much larger quantities of colorant as compared to molecularly dispersed dyes, wherein each individual molecule used in the coating contributes to coloration. Pigments are particulate solids which are difficult to handle and mix with other liquid coating components. Most pigments must be dispersed in a solvent and be specially milled (to form particles of a specific size) before they can be mixed with other coating components. The pigment particles also tend to settle out of coating preparations and require stirring to keep the color uniformly distributed throughout the coating. Particles also tend to agglomerate or flocculate, thus changing the consistency and performance of the coating. Since particulate pigments are relatively opaque, they also interfere with production of a truly transparent colored finish. Inclusion of the particulate pigments also tends to change the viscosity, rheology, and other physical characteristics of the polymer component of the coating. Finally, the pigment particles form "bumps" in the coating surface that reflect light in disparate directions, thus detracting from the desired mirror-like surface gloss of the coating.
Dyes show special utility for producing a truly transparent coloration. If a dye can be solubilized in a medium, unlike pigments, the dye molecules actually become molecularly dispersed throughout the solvent such that each molecule contributes to coloration. However, dyes have shown only limited utility in producing durable polymer coatings of the type previously described due to their volatility and poor solubility characteristics. Most dyes are practically insoluble in the types of polymers and solvents used for durable coatings. As a result, dyes have been difficult to combine with coating polymers. Although some coating mixtures containing dyes can be obtained, they tend to give non-uniform coloration due to the difficulty of achieving a true solution. In many instances the dye will actually leach, bleed or bloom out of the applied coating, resulting in loss of coloration. Prolonged exposure to heat may also volatilize the dye, thus allowing it to escape from the coating layer. For all of these reasons, dyes have not been widely used in such applications.